The present invention relates generally to electrically operated power tools and in particular, to motor controllers that control the application of power to brushless DC motors that are used to power portable power tools.
Over the past couple of decades the use of cordless power tools has increased dramatically. Cordless power tools provide the ease of a power assisted tool with the convenience of cordless operation. Generally, cordless tools are driven by a Permanent Magnet (PM) brushed motor that receives DC power from a battery assembly or converted AC power. Recently however, permanent magnet brushless DC motors have been used to cordless power tools.
In a brushless DC motor, commutation is achieved electronically by controlling the flow of current to the stator windings. A brushless DC motor is comprised of a rotor for providing rotational energy and a stator for supplying a magnetic field that drives the rotor. Comprising the rotor is a shaft supported by a bearing set on each end and encircled by a permanent magnet (PM) that generates a magnetic field. The stator core mounts around the rotor maintaining an air-gap at all points except for the bearing set interface. Included in the air-gap are sets of stator windings that are typically connected in either a three-phase wye or delta configuration. Each of the windings is oriented such that it lies parallel to the rotor shaft. Power devices such as MOSFETs are connected in series with each winding to enable power to be selectively applied. When power is applied to a winding, the resulting current in the winding generates a magnetic field that couples to the rotor. The magnetic field associated with the PM in the rotor assembly attempts to align itself with the stator generated magnetic field resulting in rotational movement of the rotor. A position sensor circuit senses the position of the rotor as the rotor rotates. In response to signals from the position sensor circuit, a control circuit sequentially activates the individual stator coils so that the PM attached to the rotor continuously chases the advancing magnetic field generated by the stator windings.
Generally, conventional portable tools motor controllers for 3-phase brushless DC motors require three position sensors. The position sensors are typically mounted so that they either detect the magnetic field peaks and nulls associated with the PM or monitor the field from a magnetic device attached to the rotor shaft. The cost of the power tool is affected by the quantity of position sensors that are employed. Costs associated with the position sensors include the cost of the sensor device, the interconnect assemblies that carry the position signals to the control circuit, and production costs associated with mounting additional devices. The consumer market for portable power tools is a highly competitive market in which cost is a major factor in the success or failure of a power tool.
Therefore, reducing the costs associated with a portable power tool is very desirable. One method of reducing the cost of brushless DC motors is to eliminate one or more position sensors. However, when a 3-phase brushless DC motor is operated with fewer than three position sensors, starting the motor becomes problematic; the position of the rotor can longer be determined unambiguously throughout all six operating states. Since the rotor position is ambiguous, the selection of the commutation switches to ensure that positive torque is applied to the rotor is problematic. If negative torque is applied, the rotor will start in the negative direction. In addition, the starting torque of the motor may be insufficient to start the motor while loaded.
The present invention provides a system and method for controlling a brushless DC motor having two or fewer position sensors. The motor includes a rotor and a stator having at least three phases. The rotor is magnetically coupled to and moveable by the stator when the phases are appropriately energized. Each of the phases is characterized by a corresponding voltage waveform. No more than two position sensors are provided for sensing the position of the rotor during a start-up mode. Each position sensor has an associated position sensor signal. The position sensors are aligned to sense the rotor position such that each position sensor signal indicates a zero torque point corresponding to a phase voltage waveform. The position of the rotor is sensed such that the position sensor signals indicate the start-up operating state of the motor. During a first ambiguous start-up state, two predetermined phases are alternately energized. During a defined start-up operating state, one predetermined phase is energized.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.